Manufacturing for virtual and augmented reality systems and components

ABSTRACT

Disclosed is an improved diffraction structure for 3D display systems. The improved diffraction structure includes an intermediate layer that resides between a waveguide substrate and a top grating surface. The top grating surface comprises a first material that corresponds to a first refractive index value, the underlayer comprises a second material that corresponds to a second refractive index value, and the substrate comprises a third material that corresponds to a third refractive index value. According to additional embodiments, improved approaches are provided to implement deposition of imprint materials onto a substrate, which allow for very precise distribution and deposition of different imprint patterns onto any number of substrate surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority to U.S.Provisional Patent Application Ser. No. 62/128,925, filed on Mar. 5,2015, which is hereby incorporated by reference in its entirety. Thepresent application is also a Continuation-in-Part of U.S. applicationSer. No. 15/007,117, filed on Jan. 26, 2016, which claims the benefit ofpriority to U.S. Provisional Patent Application Ser. No. 62/107,977,filed on Jan. 26, 2015, both of which are hereby incorporated byreference in their entirety. The present application is also related toU.S. Provisional Patent Application Ser. No. 61/909,774 filed on Nov.27, 2013 and U.S. Utility patent application Ser. No. 14/555,585 filedon Nov. 27, 2014, which are incorporated by reference herein in theirentirety. Described in the aforementioned incorporated patentapplications are various embodiments of augmented reality configurationswherein diffractive optical elements (DOE) and patterns are utilized tocreate inbound lightfields for perception by the human vision system.Described herein are further embodiments of diffractive gratingstructures and disclosure regarding their associated optical performanceand fabrication.

FIELD OF THE INVENTION

The present disclosure relates to virtual reality and augmented realityimaging and visualization systems.

BACKGROUND

Modern computing and display technologies have facilitated thedevelopment of systems for so called “virtual reality” or “augmentedreality” experiences, wherein digitally reproduced images or portionsthereof are presented to a user in a manner wherein they seem to be, ormay be perceived as, real. A virtual reality, or “VR”, scenariotypically involves presentation of digital or virtual image informationwithout transparency to actual real-world visual input. An augmentedreality, or “AR”, scenario typically involves presentation of digital orvirtual image information as an augmentation to visualization of theactual world around the user. For example, referring to FIG. 1, anaugmented reality scene (4) is depicted wherein a user of an ARtechnology sees a real-world park-like setting (6) featuring people,trees, buildings in the background, and a concrete platform (1120). Inaddition to these items, the user of the AR technology also perceivesthat he “sees” a robot statue (1110) standing upon the real-worldplatform (1120), and a cartoon-like avatar character (2) flying by whichseems to be a personification of a bumble bee, even though theseelements (2, 1110) do not exist in the real world. As it turns out, thehuman visual perception system is very complex, and producing a VR or ARtechnology that facilitates a comfortable, natural-feeling, richpresentation of virtual image elements amongst other virtual orreal-world imagery elements is challenging.

There are numerous challenges when it comes to presenting 3D virtualcontent to a user of an AR system. A central premise of presenting 3Dcontent to a user involves creating a perception of multiple depths. Inother words, it may be desirable that some virtual content appear closerto the user, while other virtual content appear to be coming fromfarther away. Thus, to achieve 3D perception, the AR system should beconfigured to deliver virtual content at different focal planes relativeto the user.

In order for a 3D display to produce a true sensation of depth, and morespecifically, a simulated sensation of surface depth, it is desirablefor each point in the display's visual field to generate theaccommodative response corresponding to its virtual depth. If theaccommodative response to a display point does not correspond to thevirtual depth of that point, as determined by the binocular depth cuesof convergence and stereopsis, the human visual system may experience anaccommodation conflict, resulting in unstable imaging, harmful eyestrain, headaches, and, in the absence of accommodation information,almost a complete lack of surface depth.

Therefore, there is a need for improved technologies to implement 3Ddisplays that resolve these and other problems of the conventionalapproaches. The systems and techniques described herein are configuredto work with the visual configuration of the typical human to addressthese challenges.

SUMMARY

Embodiments of the present invention are directed to devices, systemsand methods for facilitating virtual reality and/or augmented realityinteraction for one or more users.

An augmented reality (AR) display system for delivering augmentedreality content to a user, according to some embodiments, comprises animage-generating source to provide one or more frames of image data, alight modulator to transmit light associated with the one or more framesof image data, a diffractive optical element (DOE) to receive the lightassociated with the one or more frames of image data and direct thelight to the user's eyes, the DOE comprising a diffraction structurehaving a waveguide substrate corresponding to a waveguide refractiveindex, a surface grating, and an intermediate layer (referred to alsoherein as an “underlayer”) disposed between the waveguide substrate andthe surface grating, wherein the underlayer corresponds to an underlayerdiffractive index that is different from the waveguide refractive index.

According to some embodiments of the invention, a diffraction structureis employed for a DOE that includes an underlayer that resides between awaveguide substrate and a top grating surface. The top grating surfacecomprises a first material that corresponds to a first refractive indexvalue, the underlayer comprises a second material that corresponds to asecond refractive index value, and the substrate comprises a thirdmaterial that corresponds to a third refractive index value.

Any combination of same or different materials may be employed toimplement each of these portions of structure, e.g., where all threematerials are different (and all three correspond to differentrefractive index values), or where two of the layers share the samematerial (e.g., where two of the three materials are the same andtherefore share a common reflective index value that differs from therefractive index value of the third material). Any suitable set ofmaterials may be used to implement any layer of the improved diffractionstructure.

Thus a variety of combinations is available wherein an underlayer of oneindex is combined with a top grating of another index, along with asubstrate of a third index, and wherein adjusting these relative valuesprovides a lot of variation in dependence of diffraction efficiency uponincidence angle. A layered waveguide with different layers of refractiveindices is presented. Various combinations and permutations arepresented along with related performance data to illustratefunctionality. The benefits include increased angle, which provides anincreased output angle with the grating and therefore an increased fieldof view with the eyepiece. Further, the ability to counteract the normalreduction in diffraction efficiency with angle is functionallybeneficial.

According to additional embodiments, improved approaches are provided toimplement deposition of imprint materials onto a substrate, along withimprinting of the imprint materials to for patterns for implementingdiffraction. These approaches allow for very precise distribution,deposition, and/or formation of different imprint materials/patternsonto any number of substrate surfaces. According to some embodiments,patterned distribution (e.g., patterned inkjet distribution) of imprintmaterials are performed to implement the deposition of imprint materialsonto a substrate. This approach of using patterned ink-jet distributionallows for very precise volume control over the materials to bedeposited. In addition, this approach can serve to provide a smaller,more uniform base layer beneath a grating surface.

In some embodiments, a template is provided having a first set of deeperdepth structures along with a second set of shallower depth structures.When depositing imprint materials onto an imprint receiver, a relativelyhigher volume of imprint materials is deposited in conjunction with thedeeper depth structures of the template. In addition, a relatively lowervolume of imprint materials is deposited in conjunction with theshallower depth structures of the template. This approach permitssimultaneous deposition of different thicknesses of materials for thedifferent features to be formed onto the imprint receiver. This approachcan be taken to create distributions that are purposefully non-uniformfor structures with different depths and/or feature parameters, e.g.,where the feature structures are on the same substrate and havedifferent thicknesses. This can be used, for example, to createspatially distributed volumes of imprint material that enablesimultaneous imprint of structures of variable depth with the sameunderlayer thickness.

Some embodiments pertain to an approach to implement simultaneousdeposition of multiple types of imprint materials onto a substrate. Thispermits materials having optical properties to be simultaneouslydeposited across multiple portions of the substrate at the same time.This approach also provides the ability to tune local areas associatedwith specific functions, e.g., to act as in-coupling grating, orthogonalpupil expander (OPE) gratings, or exit pupil expander (EPE) gratings.The different types of materials may comprise the same material havingdifferent optical properties (e.g., two variants of the same materialhaving differing indices of refraction) or two entirely differentmaterials. Any optical property of the materials can be considered andselected when employing this technique, e.g., index of refraction,opacity, and/or absorption.

According to another embodiment, multi-sided imprinting may be employedto imprint multiple sides of an optical structure. This permitsimprinting to occur on different sides of an optical element, toimplement multiplexing of functions through a base layer volume. In thisway, different eyepiece functions can be implemented without adverselyaffecting grating structure function. A first template may be used toproduce one imprint on side “A” of the substrate/imprint receiver,forming a first pattern having a first material onto side A of thestructure. Another template may be used to produce a second imprint onside “B” of the same substrate, which forms a second pattern having asecond material onto side B of the substrate. Sides A and B may have thesame or different patterns, and/or may have the same or different typesof materials.

Additional embodiments pertains to multi-layer over-imprinting, and/ormulti-layer separated/offset substrate integration. In either/both ofthese approaches, a previously imprinted pattern can be jetted upon andprinted again. An adhesive can be jetted onto a first layer, with asecond substrate bonded to it (possibly with an airgap), and asubsequent jetting process can deposit onto the second substrate andimprinted. Series-imprinted patterns can be bonded to each other insequence in a roll-to-roll process. It is noted that the approach ofimplementing multi-layer over-imprinting may be used in conjunctionwith, or instead of, the multi-layer separated/offset substrateintegration approach. For multi-layer over-imprinting, a first imprintmaterial can be deposited and imprinted onto a substrate followed bydeposition of a second imprint material deposition, resulting in acomposite, multi-layer structure having both a first imprint materialand a second imprint material. For multi-layer separated/offsetsubstrate integration, both a first substrate 1 and a second substrate 2may be imprinted with the imprinting material, and afterwards, substrate1 and substrate 2 may be sandwiched and bonded, possibly with offsetfeatures (also imprinted) that provide for, in one embodiment, anair-gap between the active structures of substrate 2 and the back sideof substrate 1. An imprinted spacer may be used to create the air-gap.

According to yet another embodiment, disclosed is an approach toimplement variable volume deposition of materials distributed across thesubstrate, which may be dependent upon an apriori knowledge of surfacenon-uniformity. This corrects for surface non-uniformity of thesubstrate may result undesirable parallelism, causing poor opticalperformance. Variable volume deposition of imprint material may beemployed to provide a level distribution of imprint material to bedeposited independently of the underlying topography or physical featureset. For example, the substrate can be pulled flat by vacuum chuck, andin situ metrology performed to assess surface height, e.g., with lowcoherence or with laser based on-contact measurement probes. Thedispense volume of the imprint material can be varied depending upon themeasurement data to yield a more uniform layer upon replication. Anytypes of non-uniformity may also be addressed by this embodiment of theinvention, such as thickness variability and/or the existence of pits,peaks or other anomalies or features associated with local positions onthe substrate.

Additional and other objects, features, and advantages of the inventionare described in the detail description, figures and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a user's view of augmented reality (AR) through awearable AR user device, in one illustrated embodiment.

FIG. 2 illustrates a conventional stereoscopic 3-D simulation displaysystem.

FIG. 3 illustrates an improved approach to implement a stereoscopic 3-Dsimulation display system according to some embodiments of theinvention.

FIGS. 4A-4D illustrates various systems, subsystems, and components foraddressing the objectives of providing a high-quality,comfortably-perceived display system for human VR and/or AR.

FIG. 5 illustrates a plan view of an example configuration of a systemutilizing the improved diffraction structure.

FIG. 6 illustrates a stacked waveguide assembly.

FIG. 7 illustrates a DOE.

FIGS. 8 and 9 illustrate example diffraction patterns.

FIGS. 10 and 11 illustrate two waveguides into which a beam is injected.

FIG. 12 illustrates a stack of waveguides.

FIG. 13A illustrates an example approach to implement a diffractionstructure having a waveguide substrate and a top grating surface, butwithout an underlayer.

FIG. 13B shows a chart of example simulation results.

FIG. 13C shows an annotated version of FIG. 13A.

FIG. 14A illustrates an example approach to implement a diffractionstructure having a waveguide substrate, an underlayer, and a top gratingsurface.

FIG. 14B illustrates an example approach to implement a diffractionstructure having a waveguide substrate, an underlayer, a gratingsurface, and a top surface.

FIG. 14C illustrates an example approach to implement stacking ofdiffraction structures having a waveguide substrate, an underlayer, agrating surface, and a top surface.

FIG. 15A illustrates an example approach to implement a diffractionstructure having a high index waveguide substrate, a low indexunderlayer, and a low index top grating surface.

FIG. 15B shows charts of example simulation results.

FIG. 16A illustrates an example approach to implement a diffractionstructure having a low index waveguide substrate, a high indexunderlayer, and a low index top grating surface.

FIG. 16B shows charts of example simulation results.

FIG. 17A illustrates an example approach to implement a diffractionstructure having a low index waveguide substrate, a medium indexunderlayer, and a high index top grating surface.

FIG. 17B shows a chart of example simulation results.

FIG. 18A-D illustrate modification of underlayer characteristics.

FIG. 19 illustrates an approach to implement precise, variable volumedeposition of imprint material on a single substrate.

FIG. 20 illustrates an approach to implement directed, simultaneousdeposition of multiple different imprint materials in the same layer andimprint step according to some embodiments.

FIGS. 21A-B illustrates an example approach to implement two-sidedimprint in the context of total-internal reflection diffractive opticalelements.

FIG. 22 illustrates a structure formed using the approach shown in FIGS.21A-B.

FIG. 23 illustrates an approach to implement multi-layerover-imprinting.

FIG. 24 illustrates an approach to implement multi-layerseparated/offset substrate integration.

FIG. 25 illustrates an approach to implement variable volume depositionof materials distributed across the substrate to address surfacenon-uniformity.

DETAILED DESCRIPTION

According to some embodiments of the invention, a diffraction structureis employed that includes an underlayer/intermediate layer that residesbetween a waveguide substrate and a top grating surface. The top gratingsurface comprises a first material that corresponds to a firstrefractive index value, the underlayer comprises a second material thatcorresponds to a second refractive index value, and the substratecomprises a third material that corresponds to a third refractive indexvalue.

One advantage of this approach is that appropriate selection of therelative indices of refraction for the three layers allows the structureto obtain a larger field of view for a greater range of incident light,by virtue of the fact that the lowest total internal reflection angle isreduced as the index of refraction is increased. Diffractionefficiencies can be increased, allowing for “brighter” light outputs tothe display(s) of image viewing devices.

A variety of combinations is available wherein an underlayer of oneindex is combined with a top grating of another index, along with asubstrate of a third index, and wherein adjusting these relative valuesprovides a lot of variation in dependence of diffraction efficiency uponincidence angle. A layered waveguide with different layers of refractiveindices is presented. Various combinations and permutations arepresented along with related performance data to illustratefunctionality. The benefits include increased angle, which provides anincreased output angle with the grating and therefore an increased fieldof view with the eyepiece. Further, the ability to counteract the normalreduction in diffraction efficiency with angle is functionallybeneficial.

Display Systems According to Some Embodiments

This portion of the disclosure describes example display systems thatmay be used in conjunction with the improved diffraction structure ofthe invention.

FIG. 2 illustrates a conventional stereoscopic 3-D simulation displaysystem that typically has a separate display 74 and 76 for each eye 4and 6, respectively, at a fixed radial focal distance 10 from the eye.This conventional approach fails to take into account many of thevaluable cues utilized by the human eye and brain to detect andinterpret depth in three dimensions, including the accommodation cue.

In fact, the typical human eye is able to interpret numerous layers ofdepth based upon radial distance, e.g., able to interpret approximately12 layers of depth. A near field limit of about 0.25 meters is about theclosest depth of focus; a far-field limit of about 3 meters means thatany item farther than about 3 meters from the human eye receivesinfinite focus. The layers of focus get more and more thin as one getscloser to the eye; in other words, the eye is able to perceivedifferences in focal distance that are quite small relatively close tothe eye, and this effect dissipates as objects fall farther away fromthe eye. At an infinite object location, a depth of focus/dioptricspacing value is about 1/3 diopters.

FIG. 3 illustrates an improved approach to implement a stereoscopic 3-Dsimulation display system according to some embodiments of theinvention, where two complex images are displayed, one for each eye 4and 6, with various radial focal depths (12) for various aspects (14) ofeach image may be utilized to provide each eye with the perception ofthree dimensional depth layering within the perceived image. Since thereare multiple focal planes (e.g., 12 focal planes) between the eye of theuser and infinity, these focal planes, and the data within the depictedrelationships, may be utilized to position virtual elements within anaugmented reality scenario for a user's viewing, because the human eyeis constantly sweeping around to utilize the focal planes to perceivedepth. While this figure shows a specific number of focal planes atvarious depths, it is noted that an implementation of the invention mayuse any number of focal planes as necessary for the specific applicationdesired, and the invention is therefore not limited to devices havingonly to the specific number of focal planes shown in any of the figuresin the present disclosure.

Referring to FIGS. 4A-4D, some general componentry options areillustrated according to some embodiments of the invention. In theportions of the detailed description which follow the discussion ofFIGS. 4A-4D, various systems, subsystems, and components are presentedfor addressing the objectives of providing a high-quality,comfortably-perceived display system for human VR and/or AR.

As shown in FIG. 4A, an AR system user (60) is depicted wearing a frame(64) structure coupled to a display system (62) positioned in front ofthe eyes of the user. A speaker (66) is coupled to the frame (64) in thedepicted configuration and positioned adjacent the ear canal of the user(in one embodiment, another speaker, not shown, is positioned adjacentthe other ear canal of the user to provide for stereo/shapeable soundcontrol). The display (62) is operatively coupled (68), such as by awired lead or wireless connectivity, to a local processing and datamodule (70) which may be mounted in a variety of configurations, such asfixedly attached to the frame (64), fixedly attached to a helmet or hat(80) as shown in the embodiment of FIG. 4B, embedded in headphones,removably attached to the torso (82) of the user (60) in abackpack-style configuration as shown in the embodiment of FIG. 4C, orremovably attached to the hip (84) of the user (60) in a belt-couplingstyle configuration as shown in the embodiment of FIG. 4D.

The local processing and data module (70) may comprise a power-efficientprocessor or controller, as well as digital memory, such as flashmemory, both of which may be utilized to assist in the processing,caching, and storage of data a) captured from sensors which may beoperatively coupled to the frame (64), such as image capture devices(such as cameras), microphones, inertial measurement units,accelerometers, compasses, GPS units, radio devices, and/or gyros;and/or b) acquired and/or processed using the remote processing module(72) and/or remote data repository (74), possibly for passage to thedisplay (62) after such processing or retrieval. The local processingand data module (70) may be operatively coupled (76, 78), such as via awired or wireless communication links, to the remote processing module(72) and remote data repository (74) such that these remote modules (72,74) are operatively coupled to each other and available as resources tothe local processing and data module (70).

In one embodiment, the remote processing module (72) may comprise one ormore relatively powerful processors or controllers configured to analyzeand process data and/or image information. In one embodiment, the remotedata repository (74) may comprise a relatively large-scale digital datastorage facility, which may be available through the internet or othernetworking configuration in a “cloud” resource configuration. In oneembodiment, all data is stored and all computation is performed in thelocal processing and data module, allowing fully autonomous use from anyremote modules.

Perceptions of Z-axis difference (i.e., distance straight out from theeye along the optical axis) may be facilitated by using a waveguide inconjunction with a variable focus optical element configuration. Imageinformation from a display may be collimated and injected into awaveguide and distributed in a large exit pupil manner using anysuitable substrate-guided optics methods known to those skilled in theart—and then variable focus optical element capability may be utilizedto change the focus of the wavefront of light emerging from thewaveguide and provide the eye with the perception that the light comingfrom the waveguide is from a particular focal distance. In other words,since the incoming light has been collimated to avoid challenges intotal internal reflection waveguide configurations, it will exit incollimated fashion, requiring a viewer's eye to accommodate to the farpoint to bring it into focus on the retina, and naturally be interpretedas being from optical infinity—unless some other intervention causes thelight to be refocused and perceived as from a different viewingdistance; one suitable such intervention is a variable focus lens.

In some embodiments, collimated image information is injected into apiece of glass or other material at an angle such that it totallyinternally reflects and is passed into the adjacent waveguide. Thewaveguide may be configured so that the collimated light from thedisplay is distributed to exit somewhat uniformly across thedistribution of reflectors or diffractive features along the length ofthe waveguide. Upon exit toward the eye, the exiting light is passedthrough a variable focus lens element wherein, depending upon thecontrolled focus of the variable focus lens element, the light exitingthe variable focus lens element and entering the eye will have variouslevels of focus (a collimated flat wavefront to represent opticalinfinity, more and more beam divergence/wavefront curvature to representcloser viewing distance relative to the eye 58).

In a “frame sequential” configuration, a stack of sequentialtwo-dimensional images may be fed to the display sequentially to producethree-dimensional perception over time, in a manner akin to the mannerin which a computed tomography system uses stacked image slices torepresent a three-dimensional structure. A series of two-dimensionalimage slices may be presented to the eye, each at a different focaldistance to the eye, and the eye/brain would integrate such a stack intoa perception of a coherent three-dimensional volume. Depending upon thedisplay type, line-by-line, or even pixel-by-pixel sequencing may beconducted to produce the perception of three-dimensional viewing. Forexample, with a scanned light display (such as a scanning fiber displayor scanning mirror display), then the display is presenting thewaveguide with one line or one pixel at a time in a sequential fashion.

Referring to FIG. 6, a stacked waveguide assembly (178) may be utilizedto provide three-dimensional perception to the eye/brain by having aplurality of waveguides (182, 184, 186, 188, 190) and a plurality ofweak lenses (198, 196, 194, 192) configured together to send imageinformation to the eye with various levels of wavefront curvature foreach waveguide level indicative of focal distance to be perceived forthat waveguide level. A plurality of displays (200, 202, 204, 206, 208),or in another embodiment a single multiplexed display, may be utilizedto inject collimated image information into the waveguides (182, 184,186, 188, 190), each of which may be configured, as described above, todistribute incoming light substantially equally across the length ofeach waveguide, for exit down toward the eye.

The waveguide (182) nearest the eye is configured to deliver collimatedlight, as injected into such waveguide (182), to the eye, which may berepresentative of the optical infinity focal plane. The next waveguideup (184) is configured to send out collimated light which passes throughthe first weak lens (192; e.g., a weak negative lens) before it canreach the eye (58). The first weak lens (192) may be configured tocreate a slight convex wavefront curvature so that the eye/braininterprets light coming from that next waveguide up (184) as coming froma first focal plane closer inward toward the person from opticalinfinity. Similarly, the third up waveguide (186) passes its outputlight through both the first (192) and second (194) lenses beforereaching the eye (58). The combined optical power of the first (192) andsecond (194) lenses may be configured to create another incrementalamount of wavefront divergence so that the eye/brain interprets lightcoming from that third waveguide up (186) as coming from a second focalplane even closer inward toward the person from optical infinity thanwas light from the next waveguide up (184).

The other waveguide layers (188, 190) and weak lenses (196, 198) aresimilarly configured, with the highest waveguide (190) in the stacksending its output through all of the weak lenses between it and the eyefor an aggregate focal power representative of the closest focal planeto the person. To compensate for the stack of lenses (198, 196, 194,192) when viewing/interpreting light coming from the world (144) on theother side of the stacked waveguide assembly (178), a compensating lenslayer (180) is disposed at the top of the stack to compensate for theaggregate power of the lens stack (198, 196, 194, 192) below. Such aconfiguration provides as many perceived focal planes as there areavailable waveguide/lens pairings, again with a relatively large exitpupil configuration as described above. Both the reflective aspects ofthe waveguides and the focusing aspects of the lenses may be static(i.e., not dynamic or electro-active). In an alternative embodiment theymay be dynamic using electro-active features as described above,enabling a small number of waveguides to be multiplexed in a timesequential fashion to produce a larger number of effective focal planes.

Various diffraction configurations can be employed for focusing and/orredirecting collimated beams. For example, passing a collimated beamthrough a linear diffraction pattern, such as a Bragg grating, willdeflect, or “steer”, the beam. Passing a collimated beam through aradially symmetric diffraction pattern, or “Fresnel zone plate”, willchange the focal point of the beam. A combination diffraction patterncan be employed that has both linear and radial elements produces bothdeflection and focusing of a collimated input beam. These deflection andfocusing effects can be produced in a reflective as well as transmissivemode.

These principles may be applied with waveguide configurations to allowfor additional optical system control. As shown in FIG. 7, a diffractionpattern (220), or “diffractive optical element” (or “DOE”) has beenembedded within a planar waveguide (216) such that as a collimated beamis totally internally reflected along the planar waveguide (216), itintersects the diffraction pattern (220) at a multiplicity of locations.The structure may also include another waveguide (218) into which thebeam may be injected (by a projector or display, for example), with aDOE (221) embedded in this other waveguide (218),

Preferably, the DOE (220) has a relatively low diffraction efficiency sothat only a portion of the light of the beam is deflected toward the eye(58) with each intersection of the DOE (220) while the rest continues tomove through the planar waveguide (216) via total internal reflection;the light carrying the image information is thus divided into a numberof related light beams that exit the waveguide at a multiplicity oflocations and the result is a fairly uniform pattern of exit emissiontoward the eye (58) for this particular collimated beam bouncing aroundwithin the planar waveguide (216), as shown in FIG. 8. The exit beamstoward the eye (58) are shown in FIG. 8 as substantially parallel,because, in this case, the DOE (220) has only a linear diffractionpattern. However, changes to this linear diffraction pattern pitch maybe utilized to controllably deflect the exiting parallel beams, therebyproducing a scanning or tiling functionality.

Referring to FIG. 9, with changes in the radially symmetric diffractionpattern component of the embedded DOE (220), the exit beam pattern ismore divergent, which would require the eye to accommodation to a closerdistance to bring it into focus on the retina and would be interpretedby the brain as light from a viewing distance closer to the eye thanoptical infinity.

Referring to FIG. 10, with the addition of the other waveguide (218)into which the beam may be injected (by a projector or display, forexample), a DOE (221) embedded in this other waveguide (218), such as alinear diffraction pattern, may function to spread the light across theentire larger planar waveguide (216), which functions to provide the eye(58) with a very large incoming field of incoming light that exits fromthe larger planar waveguide (216), e.g., a large eye box, in accordancewith the particular DOE configurations at work.

The DOEs (220, 221) are depicted bisecting the associated waveguides(216, 218) but this need not be the case; they could be placed closerto, or upon, either side of either of the waveguides (216, 218) to havethe same functionality. Thus, as shown in FIG. 11, with the injection ofa single collimated beam, an entire field of cloned collimated beams maybe directed toward the eye (58). In addition, with a combined lineardiffraction pattern/radially symmetric diffraction pattern scenario suchas that discussed above, a beam distribution waveguide optic (forfunctionality such as exit pupil functional expansion; with aconfiguration such as that of FIG. 11, the exit pupil can be as large asthe optical element itself, which can be a very significant advantagefor user comfort and ergonomics) with Z-axis focusing capability ispresented, in which both the divergence angle of the cloned beams andthe wavefront curvature of each beam represent light coming from a pointcloser than optical infinity.

In one embodiment, one or more DOEs are switchable between “on” statesin which they actively diffract, and “off” states in which they do notsignificantly diffract. For instance, a switchable DOE may comprise alayer of polymer dispersed liquid crystal, in which microdropletscomprise a diffraction pattern in a host medium, and the refractiveindex of the microdroplets can be switched to substantially match therefractive index of the host material (in which case the pattern doesnot appreciably diffract incident light) or the microdroplet can beswitched to an index that does not match that of the host medium (inwhich case the pattern actively diffracts incident light). Further, withdynamic changes to the diffraction terms, a beam scanning or tilingfunctionality may be achieved. As noted above, it is desirable to have arelatively low diffraction grating efficiency in each of the DOEs (220,221) because it facilitates distribution of the light, and also becauselight coming through the waveguides that is desirably transmitted (forexample, light coming from the world 144 toward the eye 58 in anaugmented reality configuration) is less affected when the diffractionefficiency of the DOE that it crosses (220) is lower—so a better view ofthe real world through such a configuration is achieved.

Configurations such as those illustrated herein preferably are drivenwith injection of image information in a time sequential approach, withframe sequential driving being the most straightforward to implement.For example, an image of the sky at optical infinity may be injected attime1 and the diffraction grating retaining collimation of light may beutilized. Thereafter, an image of a closer tree branch may be injectedat time2 while a DOE controllably imparts a focal change, say onediopter or 1 meter away, to provide the eye/brain with the perceptionthat the branch light information is coming from the closer focal range.This kind of paradigm can be repeated in rapid time sequential fashionsuch that the eye/brain perceives the input to be all part of the sameimage. This is just a two focal plane example—preferably the system willinclude more focal planes to provide a smoother transition betweenobjects and their focal distances. This kind of configuration generallyassumes that the DOE is switched at a relatively low speed (i.e., insync with the frame-rate of the display that is injecting the images—inthe range of tens to hundreds of cycles/second).

The opposite extreme may be a configuration wherein DOE elements canshift focus at tens to hundreds of MHz or greater, which facilitatesswitching of the focus state of the DOE elements on a pixel-by-pixelbasis as the pixels are scanned into the eye (58) using a scanned lightdisplay type of approach. This is desirable because it means that theoverall display frame-rate can be kept quite low; just low enough tomake sure that “flicker” is not a problem (in the range of about 60-120frames/sec).

In between these ranges, if the DOEs can be switched at KHz rates, thenon a line-by-line basis the focus on each scan line may be adjusted,which may afford the user with a visible benefit in terms of temporalartifacts during an eye motion relative to the display, for example. Forinstance, the different focal planes in a scene may, in this manner, beinterleaved, to minimize visible artifacts in response to a head motion(as is discussed in greater detail later in this disclosure). Aline-by-line focus modulator may be operatively coupled to a line scandisplay, such as a grating light valve display, in which a linear arrayof pixels is swept to form an image; and may be operatively coupled toscanned light displays, such as fiber-scanned displays andmirror-scanned light displays.

A stacked configuration, similar to those of FIG. 6, may use dynamicDOEs to provide multi-planar focusing simultaneously. For example, withthree simultaneous focal planes, a primary focus plane (based uponmeasured eye accommodation, for example) could be presented to the user,and a + margin and − margin (i.e., one focal plane closer, one fartherout) could be utilized to provide a large focal range in which the usercan accommodate before the planes need be updated. This increased focalrange can provide a temporal advantage if the user switches to a closeror farther focus (i.e., as determined by accommodation measurement);then the new plane of focus could be made to be the middle depth offocus, with the + and − margins again ready for a fast switchover toeither one while the system catches up.

Referring to FIG. 12, a stack (222) of planar waveguides (244, 246, 248,250, 252) is shown, each having a reflector (254, 256, 258, 260, 262) atthe end and being configured such that collimated image informationinjected in one end by a display (224, 226, 228, 230, 232) bounces bytotal internal reflection down to the reflector, at which point some orall of the light is reflected out toward an eye or other target. Each ofthe reflectors may have slightly different angles so that they allreflect exiting light toward a common destination such as a pupil.Lenses (234, 236, 238, 240, 242) may be interposed between the displaysand waveguides for beam steering and/or focusing.

As discussed above, an object at optical infinity creates asubstantially planar wavefront, while an object closer, such as 1 m awayfrom the eye, creates a curved wavefront (with about 1 m convex radiusof curvature). The eye's optical system needs to have enough opticalpower to bend the incoming rays of light so that they end up focused onthe retina (convex wavefront gets turned into concave, and then down toa focal point on the retina). These are basic functions of the eye.

In many of the embodiments described above, light directed to the eyehas been treated as being part of one continuous wavefront, some subsetof which would hit the pupil of the particular eye. In another approach,light directed to the eye may be effectively discretized or broken downinto a plurality of beamlets or individual rays, each of which has adiameter less than about 0.5 mm and a unique propagation pathway as partof a greater aggregated wavefront that may be functionally created withthe an aggregation of the beamlets or rays. For example, a curvedwavefront may be approximated by aggregating a plurality of discreteneighboring collimated beams, each of which is approaching the eye froman appropriate angle to represent a point of origin that matches thecenter of the radius of curvature of the desired aggregate wavefront.

When the beamlets have a diameter of about 0.5 mm or less, it is asthough it is coming through a pinhole lens configuration, which meansthat each individual beamlet is always in relative focus on the retina,independent of the accommodation state of the eye—however the trajectoryof each beamlet will be affected by the accommodation state. Forinstance, if the beamlets approach the eye in parallel, representing adiscretized collimated aggregate wavefront, then an eye that iscorrectly accommodated to infinity will deflect the beamlets to allconverge upon the same shared spot on the retina, and will appear infocus. If the eye accommodates to, say, 1 m, the beams will be convergedto a spot in front of the retina, cross paths, and fall on multipleneighboring or partially overlapping spots on the retina—appearingblurred.

If the beamlets approach the eye in a diverging configuration, with ashared point of origin 1 meter from the viewer, then an accommodation of1 m will steer the beams to a single spot on the retina, and will appearin focus; if the viewer accommodates to infinity, the beamlets willconverge to a spot behind the retina, and produce multiple neighboringor partially overlapping spots on the retina, producing a blurred image.Stated more generally, the accommodation of the eye determines thedegree of overlap of the spots on the retina, and a given pixel is “infocus” when all of the spots are directed to the same spot on the retinaand “defocused” when the spots are offset from one another. This notionthat all of the 0.5 mm diameter or less beamlets are always in focus,and that they may be aggregated to be perceived by the eyes/brain asthough they are substantially the same as coherent wavefronts, may beutilized in producing configurations for comfortable three-dimensionalvirtual or augmented reality perception.

In other words, a set of multiple narrow beams may be used to emulatewhat is going on with a larger diameter variable focus beam, and if thebeamlet diameters are kept to a maximum of about 0.5 mm, then theymaintain a relatively static focus level, and to produce the perceptionof out-of-focus when desired, the beamlet angular trajectories may beselected to create an effect much like a larger out-of-focus beam (sucha defocussing treatment may not be the same as a Gaussian blur treatmentas for the larger beam, but will create a multimodal point spreadfunction that may be interpreted in a similar fashion to a Gaussianblur).

In some embodiments, the beamlets are not mechanically deflected to formthis aggregate focus effect, but rather the eye receives a superset ofmany beamlets that includes both a multiplicity of incident angles and amultiplicity of locations at which the beamlets intersect the pupil; torepresent a given pixel from a particular viewing distance, a subset ofbeamlets from the superset that comprise the appropriate angles ofincidence and points of intersection with the pupil (as if they werebeing emitted from the same shared point of origin in space) are turnedon with matching color and intensity, to represent that aggregatewavefront, while beamlets in the superset that are inconsistent with theshared point of origin are not turned on with that color and intensity(but some of them may be turned on with some other color and intensitylevel to represent, e.g., a different pixel).

Referring now to FIG. 5, an example embodiment 800 of the AR system thatuses an improved diffraction structure will now be described. The ARsystem generally includes an image generating processor 812, at leastone FSD 808 (fiber scanning device), FSD circuitry 810, a coupling optic832, and at least one optics assembly (DOE assembly 802) having stackedwaveguides with the improved diffraction structure described below. Thesystem may also include an eye-tracking subsystem 806. As shown in FIG.5, the FSD circuitry may comprise circuitry 810 that is in communicationwith the image generation processor 812 having a maxim chip CPU 818, atemperature sensor 820, a piezo-electrical drive/transducer 822, a redlaser 826, a blue laser 828, and a green laser 830 and a fiber combinerthat combines all three lasers 826, 828 and 830. It is noted that othertypes of imaging technologies are also usable instead of FSD devices.For example, high-resolution liquid crystal display (“LCD”) systems, abacklighted ferroelectric panel display, and/or a higher-frequency DLPsystem may all be used in some embodiments of the invention.

The image generating processor is responsible for generating virtualcontent to be ultimately displayed to the user. The image generatingprocessor may convert an image or video associated with the virtualcontent to a format that can be projected to the user in 3D. Forexample, in generating 3D content, the virtual content may need to beformatted such that portions of a particular image are displayed on aparticular depth plane while other are displayed at other depth planes.Or, all of the image may be generated at a particular depth plane. Or,the image generating processor may be programmed to feed slightlydifferent images to right and left eye such that when viewed together,the virtual content appears coherent and comfortable to the user's eyes.In one or more embodiments, the image generating processor 812 deliversvirtual content to the optics assembly in a time-sequential manner. Afirst portion of a virtual scene may be delivered first, such that theoptics assembly projects the first portion at a first depth plane. Then,the image generating processor 812 may deliver another portion of thesame virtual scene such that the optics assembly projects the secondportion at a second depth plane and so on. Here, the Alvarez lensassembly may be laterally translated quickly enough to produce multiplelateral translations (corresponding to multiple depth planes) on aframe-to frame basis.

The image generating processor 812 may further include a memory 814, aCPU 818, a GPU 816, and other circuitry for image generation andprocessing. The image generating processor may be programmed with thedesired virtual content to be presented to the user of the AR system. Itshould be appreciated that in some embodiments, the image generatingprocessor may be housed in the wearable AR system. In other embodiments,the image generating processor and other circuitry may be housed in abelt pack that is coupled to the wearable optics.

The AR system also includes coupling optics 832 to direct the light fromthe FSD to the optics assembly 802. The coupling optics 832 may refer toone more conventional lenses that are used to direct the light into theDOE assembly. The AR system also includes the eye-tracking subsystem 806that is configured to track the user's eyes and determine the user'sfocus.

In one or more embodiments, software blurring may be used to induceblurring as part of a virtual scene. A blurring module may be part ofthe processing circuitry in one or more embodiments. The blurring modulemay blur portions of one or more frames of image data being fed into theDOE. In such an embodiment, the blurring module may blur out parts ofthe frame that are not meant to be rendered at a particular depth frame.

Example approaches that can be used to implement the above image displaysystems, and components therein, are described in U.S. Utility patentapplication Ser. No. 14/555,585 filed on Nov. 27, 2014, which isincorporated by reference herein in its entirety.Improved Diffraction Structure

As stated above, a diffraction pattern can be formed onto a planarwaveguide, such that as a collimated beam is totally internallyreflected along the planar waveguide, the beam intersects thediffraction pattern at a multiplicity of locations. This arrangement canbe stacked to provide image objects at multiple focal planes within astereoscopic 3-D simulation display system according to some embodimentsof the invention.

FIG. 13A illustrates one possible approach that can be taken toimplement a structure 1300 of a waveguide 1302 (also referred to hereinas a “light guide”, “substrate”, or “waveguide substrate”), whereoutcoupling gratings 1304 are directly formed onto the top surface ofthe waveguide 1302, e.g., as a combined monolithic structure and/or bothformed of the same materials (even if not constructed out of the samemonolithic structure). In this approach, the index of refraction of thegratings material is the same as the index of refraction of thewaveguide 1302. The index of refraction n (or “refractive index”) of amaterial describes how light propagates through that medium, and isdefined as n=c/v. where c is the speed of light in vacuum and v is thephase velocity of light in the medium. The refractive index determineshow much light is bent, or refracted, when entering a material.

FIG. 13B shows a chart 1320 of example simulation results for a singlepolarization of the efficiency of the light coming out of the structure1300 as a function of the angle at which the light is propagating withinthe waveguide. This chart shows that the diffraction efficiency of theoutcoupled light for structure 1300 decreases at higher angles ofincidence. As can be seen, at an angle of about 43 degrees, theefficiency drops relatively quickly on the depicted plot due to totalinternal reflectance variation based on incident angle in a medium withuniform index of refraction.

Therefore, it is possible that the usable range of configuration 1300 issomewhat limited and therefore undesirable, as the spacing of bouncesmay decrease at higher angles of incidence, which may further reduce thebrightness seen by an observer at those angles. The diffractionefficiency is lower at the most shallow angles of incidence, which isnot entirely desirable, because the bounce spacing (see FIG. 13C)between interactions with the top surface is fairly far apart, and lighthas fairly few opportunities to couple out. Thus, a dimmer signal withfewer outcoupled samples will result from this arrangement, with thisproblem being compounded by the grating having lower diffractionefficiencies at these high angles with this polarization orientation. Itis noted that as used herein and in the figures, “1T” refers to the1^(st) transmitted diffracted order.

In some embodiments of waveguide-based optical systems or substrateguided optical systems, such as those described above, different pixelsin a substrate-guided image are represented by beams propagating atdifferent angles within the waveguide, where light propagates along thewaveguide by total internal reflection (TIR). The range of beam anglesthat remain trapped in a waveguide by TIR is a function of thedifference in refractive index between the waveguide and the medium(e.g., air) outside the waveguide; the higher the difference inrefractive index, the larger the number of beam angles. In certainembodiments, the range of beam angles propagating along the waveguidecorrelates with the field of view of the image coupled out of the faceof the waveguide by a diffractive element, and with the image resolutionsupported by the optical system. Additionally, the angle range in whichtotal internal reflection occurs is dictated by the index of refractionof the waveguide—in some embodiments a minimum of about 43 degrees and apractical maximum of approximately 83 degrees, thus a 40 degree range.

FIG. 14A illustrates an approach to address this issue according to someembodiments of the invention, where structure 1400 includes anintermediate layer 1406 (referred to herein as “underlayer 1406”) thatresides between the substrate 1302 and the top grating surface 1304. Thetop surface 1304 comprises a first material that corresponds to a firstrefractive index value, the underlayer 1406 comprises a second materialthat corresponds to a second refractive index value, and the substrate1302 comprises a third material that corresponds to a third refractiveindex value. It is noted that any combination of same or differentmaterials may be employed to implement each of these portions ofstructure 1400, e.g., where all three materials are different (and allthree correspond to different refractive index values), or where two ofthe layers share the same material (e.g., where two of the threematerials are the same and therefore share a common reflective indexvalue that differs from the refractive index value of the thirdmaterial). Any combination of refractive index values may be employed.For example, one embodiment comprises a low refractive index for theunderlayer, with higher index values for the surface grating and thesubstrate. Other example configurations are described below having otherillustrative combinations of refractive index values. Any suitable setof materials may be used to implement structure 1500. For example,polymers, glass, and sapphire are all examples of materials that can beselected to implement any of the layers of structure 1400.

As shown in FIG. 15A, in some embodiments it may be desirable toimplement a structure 1500 that uses a relatively higher refractiveindex substrate as waveguide substrate 1302, with a relatively lowerrefractive index underlayer 1406 and relatively lower refractive indextop grating surface 1304. This is because one may be able to obtain alarger field of view by virtue of the fact that the lowest totalinternal reflection angle is reduced as the index of refraction isincreased through the relationship n1*sin(theta1)=n2*sin(90). For asubstrate of index 1.5, the critical angle is 41.8 degrees; however, fora substrate index of 1.7, the critical angle is 36 degrees.

Gratings formed on higher index substrates may be utilized to couplelight out even if they themselves have a lower index of refraction, solong as the layer of material comprising the grating is not too thickbetween the grating and the substrate. This is related to the fact thatone can have a more broad range of angles for total internal reflection(“TIR”) with such a configuration. In other words, the TIR angle dropsto lower values with such a configuration. In addition, it is noted thatmany of the current etching processes may not be well suited forextending to high-index glasses. It is desirable in some embodiments toreplicate an outcoupling layer reliably and inexpensively.

The configuration of the underlayer 1406 may be adjusted to alter theperformance characteristics of structure 1500, e.g., by changing thethickness of the underlayer 1406. The configuration of FIG. 15A (aconstruct including a grating structure 1304 on top comprising arelatively low index material, with an associated lower index underlayer1406, and which also includes an associated high-index light guidingsubstrate 1302) may be modelled to result in data such as that depictedin FIG. 15B. Referring to this figure, the plot 1502 a on the left isrelated to a configuration with zero-thickness underlayer 1502. Themiddle plot 1502 b shows data for a 0.05 micron thickness underlayer1502. The right plot 1502 c shows data for a 0.1 micron thicknessunderlayer 1502.

As shown by the data in these plots, as the underlayer thickness isincreased, the diffraction efficiency as a function of incident anglebecomes much more nonlinear and suppressed at high angles, which may notbe desirable. Thus in this case, control of the underlayer is animportant functional input. However, it should be noted that with azero-thickness underlayer and only grating features themselvespossessing the lower index, the range of angles supported by thestructure is governed by the TIR condition in the higher index basematerial, rather than the lower index grating feature material.

Referring to FIG. 16A, an embodiment of a structure 1600 is illustratedfeaturing a relatively high index underlayer 1406 on a lower indexsubstrate 1302, with a top surface diffraction grating 1304 having arefractive index lower than the underlayer 1406 and comparable to, butnot necessarily equal to, the refractive index of the substrate 1302.For example, the top surface grating may correspond to a refractiveindex of 1.5, the underlayer may correspond to a refractive index of1.84, and the substrate may correspond to a refractive index of 1.5.Assume for this example that the period is 0.43 um and lambdacorresponds to 0.532 um.

Simulations related to such a configuration are presented in FIG. 16B.As shown in this figure in chart 1602 a, with a 0.3 micron thickunderlayer 1406, diffraction efficiency falls off like the previouslydescribed configuration, but then starts to rise up at the higher end ofthe angular range. This is also true for the 0.5 micron thick underlayer1406 configuration, as shown in chart 1602 b. It is beneficial in eachof these (0.3 micron, 0.5 micron) configurations, that the efficiency isrelatively high at the higher extremes of the angular range; suchfunctionality may tend to counteract the more sparse bounce spacingconcern discussed above. Also shown in this figure is chart 1602 c foran embodiment featuring a 90 degree rotated polarization case, where thediffraction efficiency is lower as might be expected, but showsdesirable behavior in that it provides greater efficiency at steeperangles as compared to shallower angles.

Indeed, in some embodiments, diffraction efficiency versus angles mayincrease at high angles. This may be a desirable feature for someembodiments since it helps to compensate for the lower bounce spacingthat may occur at higher propagation angles. Therefore, the structuralconfiguration of FIG. 16A may be preferable in embodiments where it isdesirable to compensate for the lower bounce spacing (which occurs withhigher propagation angles), since it promotes diffraction efficiencyversus angle increasing at higher angles, which is desirable relative tothe aforementioned monolithic configurations.

Referring to FIG. 17A, another structure 1700 is depicted wherein anunderlayer 1406 has a refractive index substantially higher than therefractive index of the substrate 1302. A grating structure 1304 is ontop, and has a refractive index that is also higher than the refractiveindex of the underlayer 1406. For example, the top surface grating maycorrespond to a refractive index of 1.86, the underlayer may correspondto a refractive index of 1.79, and the substrate may correspond to arefractive index of 1.5. As before, assume for this example that theperiod is 0.43 um and lambda corresponds to 0.532 um.

Referring to FIG. 17B, chart 1702 shows simulation data is illustratedfor the structure 1700 of FIG. 17A. As shown in chart 1702, the plot ofthe resulting diffraction efficiency versus incident angle demonstratesa desirable general behavior to assist in compensating for theaforementioned lower bounce spacing at relatively high incident anglesand possessing reasonable diffraction efficiency across a greater rangeof angles in general.

It is noted that the underlayer 1406 does not need to be uniform acrossthe entire substrate. Any characteristic of the underlayer 1406 may bevaried at different locations of the substrate, such as variances in thethickness, composition, and/or index of refraction of the underlayer1406. One possible reason for varying the characteristics of theunderlayer 1406 is to promote uniform display characteristics in thepresence of known variations in either the display image and/ornon-uniform transmission of light within the display system.

For example, as shown in FIG. 18A, consider if the waveguide structurereceives incoming light at a single incoupling location 1802 on thewaveguide. As the incoming light is injected into the waveguide 1302,less and less of that light will be remain as it progresses along thelength of the waveguide 1302. This means that the output light near theincoupling location 1802 may end up appearing “brighter” than outputlight farther along the length of the waveguide 1302. If the underlayer1406 is uniform along the entire length of the waveguide 1302, then theoptical effects of the underlayer 1406 may reinforce this unevenbrightness level across the substrate.

The characteristics of the underlayer 1406 can be adjusted across thesubstrate 1302 to make the output light more uniform. FIG. 18Billustrates an approach whereby the thickness of the underlayer 1406 isvaried across the length of the waveguide substrate 1302, where theunderlayer 1406 is thinner near the incoupling location 1802 and thickerat farther distances away from location 1802. In this manner, the effectof the underlayer 1406 to promote greater diffraction efficiency can atleast partially ameliorate the effects of light losses along the lengthof the waveguide substrate 1302, thereby promoting more uniform lightoutput across the entirety of the structure.

FIG. 18C illustrates an alternate approach where the thickness of theunderlayer 1406 is not varied, but the refractive index of theunderlayer 1406 varies across the substrate 1302. For example, toaddress the issue that output light near location 1802 tends to bebrighter than locations farther away from location 1802, the index ofrefraction for the underlayer 1406 can be configured to be the same orsimilar to the substrate 1302 close to location 1802, but to have anincreasing difference in those index values at locations farther awayfrom location 1802. The composition of the underlayer 1406 material canbe varied at different location to effect the different refractive indexvalues. FIG. 18D illustrates a hybrid approach, whereby both thethickness and the refractive index of the underlayer 1406 is variedacross the substrate 1302. It is noted that this same approach can betaken to vary the thickness and/or refractive index of the top gratingsurface 1304 and/or the substrate 1302 in conjunction with, or insteadof, varying the underlayer 1406.

Thus a variety of combinations is available wherein an underlayer 1406of one index is combined with a top grating 1304 of another index, alongwith a substrate 1302 of a third index, and wherein adjusting theserelative values provides a lot of variation in dependence of diffractionefficiency upon incidence angle. A layered waveguide with differentlayers of refractive indices is presented. Various combinations andpermutations are presented along with related performance data toillustrate functionality. The benefits include increased angle, whichprovides an increased output angle with the grating 1304 and thereforean increased field of view with the eyepiece. Further, the ability tocounteract the normal reduction in diffraction efficiency with angle isfunctionally beneficial.

FIG. 14B illustrates an embodiment where another layer of material 1409(top surface) is placed above the grating layer 1304. Layer 1409 can beconfigurably implemented to address different design goals. For example,layer 1409 can form an interstitial layer between multiple stackeddiffraction structures 1401 a and 1401 b, e.g., as shown in FIG. 14C. Asshown in FIG. 14C, this interstitial layer 1409 can be employed toremove any air space/gap and provide a support structure for the stackeddiffraction components. In this use case, the layer 1409 can be formedfrom a material having a relatively low index of refraction, e.g., ataround 1.1 or 1.2. Although not shown in this figure, other layers (suchas weak lenses) may also be placed between the diffraction structures1401 a and 1401 b.

In addition, layer 1409 can be formed from a material having arelatively high index of refraction. In this situation, it is thegratings on the layer 1409 that would provide the diffraction effectsfor all or a substantial amount of the incident light, rather than thegrating surface 1304.

As is clear, different relative combinations of refractive index valuescan be selected for the different layers, including layer 1409, toachieve desired optical effects and results.

Such structures may be manufactured using any suitable manufacturingtechniques. Certain high-refractive index polymers such as one known as“MR 174” may be directly embossed, printed, or etched to produce desiredpatterned structures, although there may be challenges related to cureshrinkage and the like of such layers. Thus, in another embodiment,another material may be imprinted, embossed, or etched upon ahigh-refractive index polymer layer (i.e., such as a layer of MR 174) toproduce a functionally similar result. Current state of the artprinting, etching (i.e., which may include resist removal and patterningsteps similar to those utilized in conventional semiconductorprocesses), and embossing techniques may be utilized and/or combined toaccomplish such printing, embossing, and/or etching steps. Moldingtechniques, similar to those utilized, for example, in the production ofDVDs, may also be utilized for certain replication steps. Further,certain jetting or deposition techniques utilized in printing and otherdeposition processes may also be utilized for depositing certain layerswith precision.

The following portion of the disclosure will now describe improvedapproaches to implement the formation patterns onto substrates fordiffraction, wherein imprinting of deposited imprint materials isperformed according to some embodiments of the invention. Theseapproaches allow for very precise distribution of imprint materials, aswell as very precise formation of different imprint patterns onto anynumber of substrate surfaces. It is noted that the following descriptioncan be used in conjunction with, and to implement, the gratingconfigurations described above. However, it is expressly noted that theinventive deposition approach may also be used in conjunction with otherconfigurations as well.

According to some embodiments, patterned distribution (e.g., patternedinkjet distribution) of imprint materials are performed to implement thedeposition of imprint materials onto a substrate. This approach of usingpatterned ink-jet distribution allows for very precise volume controlover the materials to be deposited. In addition, this approach can serveto provide a smaller, more uniform base layer beneath a gratingsurface—and as discussed above, the base thickness of a layer can have asignificant effect on the performance of an eyepiece/optical device.

FIG. 19 illustrates an approach to implement precise, variable volumedeposition of imprint material on a single substrate. As shown in thefigure, a template 1902 is provided having a first set of deeper depthstructures 1904 and a second set of shallower (e.g., standard) depthstructures 1906. When depositing imprint materials onto an imprintreceiver 1908, a relatively higher volume of imprint materials 1910 isdeposited in correspondence to the portion of the template with thedeeper depth structures 1904 of the template 1902. In contrast, arelatively lower volume of imprint materials 1912 is deposited inconjunction with the shallower depth structures 1906 of the template1902. The template then is used to imprint the first and second set ofdepth structures into the imprint materials, forming respectivestructures having different depths and/or patterns within the imprintmaterials. This approach therefore permits simultaneous formation ofdifferent features onto the imprint receiver 1908.

This approach can be taken to create distributions that are purposefullynon-uniform for structures with different depths and/or featureparameters, e.g., where the feature structures are on the same substrateand have different thicknesses. This can be used, for example, to createspatially distributed volumes of imprint material that enablesimultaneous imprint of structures of variable depth with the sameunderlayer thickness.

The bottom of FIG. 19 illustrates a structure 1920 that is formed withthe deposition technique/apparatus described above, where the underlayer1922 has a uniform thickness despite pattern depth and volumedifferentials. It can be seen that imprint materials have been depositedwith non-uniform thicknesses in structure 1920. Here, the top layer 1924includes a first portion 1926 having a first set of layer thicknesses,while a second portion 1928 has a second set of layer thicknesses. Inthis example, portion 1926 corresponds to a thicker layer as compared tothe standard/shallower thickness of portion 1928. It is noted, however,that any combination of thicknesses may be constructed using theinventive concept, where thicknesses that are either/both thicker and/orthinner than standard thicknesses are formed onto an underlayer.

This capability can also be used to deposit larger volumes of materialto serve as, for example, spacer elements to aid in the construction ofa multi-layer diffractive optical element, for example.

Some embodiments pertain to an approach to implement simultaneousdeposition of multiple types of imprint materials onto a substrate. Thispermits materials having optical properties to be simultaneouslydeposited across multiple portions of the substrate at the same time.This approach also provides the ability to tune local areas associatedwith specific functions, e.g., to act as in-coupling grating, orthogonalpupil expander (OPE) gratings, or exit pupil expander (EPE) gratings.

FIG. 20 illustrates an approach to implement directed, simultaneousdeposition of multiple different imprint materials in the same layer andimprint step according to some embodiments. As shown in the figure, atemplate 2002 is provided to imprint patterns into the different typesof imprint materials 2010 and 2012 on the imprint receiver 2008.Materials 2010 and 2012 may comprise the same material having differentoptical properties (e.g., two variants of the same material havingdiffering indices of refraction) or two entirely different materials.

Any optical property of the materials can be considered and selectedwhen employing this technique. For example, as shown in the embodimentof FIG. 20, material 2010 corresponds to a high index of refractionmaterial that is deposited in one section of the imprint receiver 2008,while at the same time, material 2012 corresponding to a lower index ofrefraction material that is deposited in the area of a second section.

As shown in the resulting structure 2020, this forms a multi-functiondiffractive optical element having a high index of refraction portion2026 and a lower index of refraction portion 2028. In this case, highindex portion 2026 pertaining to a first function and portion 2028pertaining to a second function were imprinted simultaneously.

While this example illustratively identifies the refractive index of thematerials as the optical property to “tune” when simultaneouslydepositing the materials, it is noted that other optical properties mayalso be considered when identifying the type of materials to deposit indifferent portions of the structure. For example, opacity and absorptionare other properties that can be used to identify materials fordeposition in different portions of the structure to tune the localproperties of the final product.

In addition, one type of material may be deposited above/below anothermaterial before imprinting. For example, one index of refractionmaterial may be deposited directly below a second index of refractionmaterial just prior to imprinting, producing a gradient index to form adiffractive optical element. This can be used, for example, to implementthe structure shown in FIG. 17A (or any of the other pertinentstructures described above or in the figures).

According to another embodiment, multi-sided imprinting may be employedto imprint multiple sides of an optical structure. This permitsimprinting to occur on different sides of an optical element, toimplement multiplexing of functions through a base layer volume. In thisway, different eyepiece functions can be implemented without adverselyaffecting grating structure function.

FIGS. 21A-B illustrates an example approach to implement two-sidedimprint in the context of total-internal reflection diffractive opticalelements. As illustrated in FIG. 21A, a first template 2102 a may beused to produce one imprint on side “A” of the substrate/imprintreceiver 2108. This forms a first pattern 2112 having a first materialonto side A of the structure.

As illustrated in FIG. 21B, template 2102 b may be used to produce asecond imprint on side “B” of the same substrate. This forms a secondpattern 2114 having a second material onto side B of the substrate.

It is noted that sides A and B may have the same or different patterns,and/or may have the same or different types of materials. In addition,the pattern on each side may comprise varying layer thicknesses (e.g.,using the approach of FIG. 19) and/or have different material types onthe same side (e.g., using the approach of FIG. 20).

As shown in FIG. 22, a first pattern 2112 has been imprinted onto side Aand a second pattern 2114 onto the opposite side B of the substrate2108. The compound function of the resulting two-sided imprinted element2200 can now be realized. In particular, when input light is applied tothe two-sided imprinted element 2200, some the light exits from theelement 2200 to implement a first function 1 while other light exits toimplement a second function 2.

Additional embodiments pertain to multi-layer over-imprinting, and/ormulti-layer separated/offset substrate integration. In either/both ofthese approaches, a previously imprinted pattern can be jetted upon andprinted again. An adhesive can be jetted onto a first layer, with asecond substrate bonded to it (possibly with an airgap), and asubsequent jetting process can deposit onto the second substrate andimprinted. Series-imprinted patterns can be bonded to each other insequence in a roll-to-roll process. It is noted that the approach ofimplementing multi-layer over-imprinting may be used in conjunctionwith, or instead of, the multi-layer separated/offset substrateintegration approach.

FIG. 23 illustrates an approach to implement multi-layerover-imprinting. Here, a first imprint material 2301 can be depositedonto a substrate 2308 and imprinted. This is followed by deposition (andpossible imprinting) of a second imprint material 2302. This results ina composite, multi-layer structure having both a first imprint material2301 and a second imprint material 2302. In one embodiment, subsequentimprinting may be implemented for the second imprint material 2302. Inan alternate embodiment, subsequent imprinting is not implemented forthe second imprint material 2302.

FIG. 24 illustrates an approach to implement multi-layerseparated/offset substrate integration. Here, both a first substrate 1and a second substrate 2 may be deposited with the imprinting materialand then imprinted. Afterwards, substrate 1 and substrate 2 may besandwiched and bonded, possibly with offset features (also imprinted)that provide for, in one embodiment, an air-gap 2402 between the activestructures of Substrate 2 and the back side of substrate 1. An imprintedspacer 2404 may be used to create the airgap 2402.

According to yet another embodiment, disclosed is an approach toimplement variable volume deposition of materials distributed across thesubstrate, which may be dependent upon an apriori knowledge of surfacenon-uniformity. To explain, consider the substrate 2502 shown in FIG.25. As shown, the surface non-uniformity of the substrate 2502 mayresult undesirable parallelism, causing poor optical performance. Inthis case, the substrate 2502 (or a previously imprinted layer) may bemeasured for variability.

Variable volume deposition of imprint material may be employed toprovide a level distribution of imprint material to be depositedindependently of the underlying topography or physical feature set. Forexample, the substrate can be pulled flat by vacuum chuck, and in situmetrology performed to assess surface height, e.g., with low coherenceor with laser based on-contact measurement probes. The dispense volumeof the imprint material can be varied depending upon the measurementdata to yield a more uniform layer upon replication. In this example,portion 2504 a of the substrate has the greatest level of variability,portion 2504 b has a medium level of variability, and portion 2504 c hasthe lowest level of variability. Therefore, high volume imprint materialmay be deposited in portion 2504 a, medium volume imprint material isdeposited into portion 2504 b, and low/standard volume imprint materialis deposited into portion 2504 c. As shown by the resulting product2506, this results in a more uniform total substrate/imprintmaterial/imprint pattern thickness, which may in turn tune or benefitperformance of the imprinted device.

It is noted that while the example shows the variability due tonon-uniformity in thickness, other types of non-uniformity may also beaddressed by this embodiment of the invention. In another embodimentthat variability may be due to existence of pits, peaks or otheranomalies or features associated with local positions on the substrate.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Forexample, the above-described process flows are described with referenceto a particular ordering of process actions. However, the ordering ofmany of the described process actions may be changed without affectingthe scope or operation of the invention. The specification and drawingsare, accordingly, to be regarded in an illustrative rather thanrestrictive sense.

Various example embodiments of the invention are described herein.Reference is made to these examples in a non-limiting sense. They areprovided to illustrate more broadly applicable aspects of the invention.Various changes may be made to the invention described and equivalentsmay be substituted without departing from the true spirit and scope ofthe invention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processact(s) or step(s) to the objective(s), spirit or scope of the presentinvention. Further, as will be appreciated by those with skill in theart that each of the individual variations described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentinventions. All such modifications are intended to be within the scopeof claims associated with this disclosure.

The invention includes methods that may be performed using the subjectdevices. The methods may comprise the act of providing such a suitabledevice. Such provision may be performed by the end user. In other words,the “providing” act merely requires the end user obtain, access,approach, position, set-up, activate, power-up or otherwise act toprovide the requisite device in the subject method. Methods recitedherein may be carried out in any order of the recited events which islogically possible, as well as in the recited order of events.

Example aspects of the invention, together with details regardingmaterial selection and manufacture have been set forth above. As forother details of the present invention, these may be appreciated inconnection with the above-referenced patents and publications as well asgenerally known or appreciated by those with skill in the art. The samemay hold true with respect to method-based aspects of the invention interms of additional acts as commonly or logically employed.

In addition, though the invention has been described in reference toseveral examples optionally incorporating various features, theinvention is not to be limited to that which is described or indicatedas contemplated with respect to each variation of the invention. Variouschanges may be made to the invention described and equivalents (whetherrecited herein or not included for the sake of some brevity) may besubstituted without departing from the true spirit and scope of theinvention. In addition, where a range of values is provided, it isunderstood that every intervening value, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range, is encompassed within the invention.

Also, it is contemplated that any optional feature of the inventivevariations described may be set forth and claimed independently, or incombination with any one or more of the features described herein.Reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin claims associated hereto, the singular forms “a,” “an,” “said,” and“the” include plural referents unless the specifically stated otherwise.In other words, use of the articles allow for “at least one” of thesubject item in the description above as well as claims associated withthis disclosure. It is further noted that such claims may be drafted toexclude any optional element. As such, this statement is intended toserve as antecedent basis for use of such exclusive terminology as“solely,” “only” and the like in connection with the recitation of claimelements, or use of a “negative” limitation.

Without the use of such exclusive terminology, the term “comprising” inclaims associated with this disclosure shall allow for the inclusion ofany additional element—irrespective of whether a given number ofelements are enumerated in such claims, or the addition of a featurecould be regarded as transforming the nature of an element set forth insuch claims. Except as specifically defined herein, all technical andscientific terms used herein are to be given as broad a commonlyunderstood meaning as possible while maintaining claim validity.

The breadth of the present invention is not to be limited to theexamples provided and/or the subject specification, but rather only bythe scope of claim language associated with this disclosure.

The above description of illustrated embodiments is not intended to beexhaustive or to limit the embodiments to the precise forms disclosed.Although specific embodiments of and examples are described herein forillustrative purposes, various equivalent modifications can be madewithout departing from the spirit and scope of the disclosure, as willbe recognized by those skilled in the relevant art. The teachingsprovided herein of the various embodiments can be applied to otherdevices that implement virtual or AR or hybrid systems and/or whichemploy user interfaces, not necessarily the example AR systems generallydescribed above.

For instance, the foregoing detailed description has set forth variousembodiments of the devices and/or processes via the use of blockdiagrams, schematics, and examples. Insofar as such block diagrams,schematics, and examples contain one or more functions and/oroperations, it will be understood by those skilled in the art that eachfunction and/or operation within such block diagrams, flowcharts, orexamples can be implemented, individually and/or collectively, by a widerange of hardware, software, firmware, or virtually any combinationthereof.

In one embodiment, the present subject matter may be implemented viaApplication Specific Integrated Circuits (ASICs). However, those skilledin the art will recognize that the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in standard integratedcircuits, as one or more computer programs executed by one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs executed by on one or more controllers(e.g., microcontrollers) as one or more programs executed by one or moreprocessors (e.g., microprocessors), as firmware, or as virtually anycombination thereof, and that designing the circuitry and/or writing thecode for the software and or firmware would be well within the skill ofone of ordinary skill in the art in light of the teachings of thisdisclosure.

When logic is implemented as software and stored in memory, logic orinformation can be stored on any computer-readable medium for use by orin connection with any processor-related system or method. In thecontext of this disclosure, a memory is a computer-readable medium thatis an electronic, magnetic, optical, or other physical device or meansthat contains or stores a computer and/or processor program. Logicand/or the information can be embodied in any computer-readable mediumfor use by or in connection with an instruction execution system,apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions associated with logic and/or information.

In the context of this specification, a “computer-readable medium” canbe any element that can store the program associated with logic and/orinformation for use by or in connection with the instruction executionsystem, apparatus, and/or device. The computer-readable medium can be,for example, but is not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus or device.More specific examples (a non-exhaustive list) of the computer readablemedium would include the following: a portable computer diskette(magnetic, compact flash card, secure digital, or the like), a randomaccess memory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM, EEPROM, or Flash memory), a portable compactdisc read-only memory (CDROM), digital tape, and other nontransitorymedia.

Many of the methods described herein can be performed with variations.For example, many of the methods may include additional acts, omit someacts, and/or perform acts in a different order than as illustrated ordescribed.

The various embodiments described above can be combined to providefurther embodiments. To the extent that they are not inconsistent withthe specific teachings and definitions herein, all of the U.S. patents,U.S. patent application publications, U.S. patent applications, foreignpatents, foreign patent applications and non-patent publicationsreferred to in this specification and/or listed in the Application DataSheet. Aspects of the embodiments can be modified, if necessary, toemploy systems, circuits and concepts of the various patents,applications and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

Moreover, the various embodiments described above can be combined toprovide further embodiments. Aspects of the embodiments can be modified,if necessary to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

What is claimed is:
 1. A method for manufacturing an eyepiece comprisinga first diffractive optical element, comprising: depositing a firstlayer onto a first substrate, wherein the first layer comprises a firstportion and a second portion, the first portion is deposited to have afirst depth onto a first region on the first substrate, the firstportion has a first optical index, the second portion is deposited tohave a second depth onto a second region on the first substrate, and thesecond portion has a second optical index different from the firstoptical index; identifying a template having an imprint pattern formedthereon, the template comprising a first set of depth structures and asecond set of depth structures, the first set of depth structurescorresponding to the first depth of the first portion, and the secondset of depth structures corresponding to the second depth of the secondportion; and imprinting the imprint pattern into the first and thesecond portions on the first substrate with the template, wherein theimprint pattern comprises a diffraction pattern for the firstdiffractive optical element, the first portion is deposited to have thefirst depth according to at least a first optical function, the secondportion is deposited to have the second depth according to at least asecond optical function, before imprinting, and the first depth isdifferent from the second depth.
 2. The method of claim 1, furthercomprising: imprinting, using at least the template, the first andsecond portions that are respectively deposited onto the first andsecond regions simultaneously to form a first pattern and a secondpattern onto the first substrate, wherein the imprint pattern comprisesthe first pattern and the second pattern, the template imprints thefirst pattern with the first set of depth structures onto the firstportion of the materials, and the template imprints the second patternwith the second set of depth structures onto the second portion of thematerials.
 3. The method of claim 2, wherein the first patterncorresponds to a first diffraction grating pattern and the secondpattern corresponds to a second diffraction grating pattern.
 4. Themethod of claim 2, wherein the first function or the second functionincludes a first function of in-coupling gratings, a second function oforthogonal pupil expander gratings, or a third function of exit pupilexpander gratings.
 5. The method of claim 1, further comprising:depositing, for the first diffractive optical element of the eyepiece, asecond layer above the first layer, wherein the second layer comprises afirst material having a first refractive index value in a first regionof the second layer, the second layer further comprises a secondmaterial having a second refractive index value in a second region ofthe second layer, the first substrate comprises a third material havinga third refractive index value, and the first, second, and thirdrefractive index values are selected based at least in part upon a firstrequirement for a diffraction efficiency or a second requirement for afield of view provided by the first diffractive optical element of theeye piece.
 6. The method of claim 1, further comprising: stacking, forthe eyepiece, a second diffractive optical element above a firstnegative lens that is further stacked above the first diffractiveoptical element, wherein the first diffractive optical element is closerto a viewer's eye than the second diffractive optical element anddefines a first focal plane with a first focal distance of opticalinfinity, the second diffractive element comprises a second substrateand defines a second focal plane with a second focal distance that issmaller than the first focal distance of the optical infinity, and thesecond diffractive element is separated from the first diffractiveelement by the first lens.
 7. The method of claim 6, further comprising:stacking, for the eyepiece, a third diffractive optical element abovethe second diffractive optical element, wherein the third diffractiveoptical element comprises a third substrate and is disposed farther awayfrom the viewer's eye than the second diffractive optical element, thethird diffractive optical element defines a third focal plane with athird focal distance that is smaller than the second focal distance, andthe third diffractive optical element is separated from the seconddiffractive element by a second lens.
 8. The method of claim 7, furthercomprising: disposing, for the eyepiece, a compensation lens layer abovethe third diffractive optical element, wherein the compensation lenslayer is selected based at least in part upon an aggregate power of thefirst and the second lens.
 9. The method of claim 7, wherein the firstlens and the second lens create wavefront divergence to define adifferent focal plane with a different focal distance that is smallerthan the third focal distance.
 10. The method of claim 7, furthercomprising stacking, for the eyepiece, one or more additionaldiffractive optical elements above the third diffractive optical elementand away from the viewer's eye, wherein the third diffractive element isseparated from the one or more additional diffractive optical elementsby a separate lens, and the one or more diffractive optical elementsrespectively define respective focal planes with one or morecorresponding focal distances that are smaller than the third focaldistance.
 11. The method of claim 10, further comprising: disposing acompensation lens layer above the one or more additional diffractiveoptical elements, wherein the compensation lens layer is selected basedat least in part upon an aggregate power of lenses separatingdiffractive optical layers in the eyepiece.
 12. The method of claim 7,wherein at least two of the first diffractive optical element, thesecond diffractive optical element, and the third diffractive opticalelement are multiplexed to create at least one additional focal plane inaddition to the first focal plane, the second focal plane, and the thirdfocal plane.
 13. The method of claim 6, wherein at least some of thefirst portion of the materials is deposited above at least some of thesecond portion of the materials before imprinting the imprint patterninto the first and the second portions.
 14. The method of claim 6,wherein the first diffractive optical element corresponds to a first setof one or more projectors, and the second diffractive optical elementcorresponds to a second set of one or more projectors.
 15. The method ofclaim 1, further comprising: imprinting a first pattern of the imprintpattern into the first portion; and imprinting a second pattern into thesecond portion that is different from the first pattern, wherein theimprint pattern comprises the first pattern and the second pattern. 16.The method of claim 1, wherein the first substrate having a firstimprinted pattern is overlaid onto a second substrate having a secondimprinted pattern.
 17. The method of claim 1, further comprising:depositing, for the first diffractive optical element of the eyepiece, asecond layer onto the first substrate, wherein the second layer isdeposited on an opposite surface opposing a surface onto which the firstlayer is deposited, the second layer comprises a first material having arefractive index value in a first region of the second layer, the firstsubstrate comprises a different material having a different refractiveindex value, and the first and the third refractive index values areselected based at least in part upon a first requirement for adiffraction efficiency or a second requirement for a field of viewprovided by the first diffractive optical element of the eye piece. 18.The method of claim 1, wherein the first refractive index value issmaller than the second refractive index value and a substraterefractive index value of the first substrate, and the substraterefractive index value is smaller than the second refractive indexvalue.
 19. The method of claim 1, further comprising determining athickness value for the first layer based at least in part upon a lightincident angle for the first layer, the first refractive index value,and the second refractive value.